The present invention relates to a signal transmission/reception system of orthogonal frequency division multiplexing, and more particularly, to a signal transmission/reception system of orthogonal frequency division multiplexing based on synchronous modulation such as a 64 QAM (Quadrature Amplitude Modulation) scheme.
In recent years, in the field of radio devices, the OFDM scheme has been in the limelight as a modulation scheme immune to multipath fading. A large number of applied studies on the OFDM are now under progress in the fields of next-generation television broadcasting, FPU (Field Pickup Unit), radio LAN and so on in many countries including European countries and Japan.
Here, the OFDM (Orthogonal Frequency Division Multiplexing) scheme is an acronym for an orthogonal frequency division multiplexing modulation in which information codes are transmitted by using a plurality of carriers orthogonal to one another. The trends of developments in OFDM-based UHF-band terrestrial digital broadcasting and associated schemes are disclosed in detail in “The Journal of the Institute of Image Information and Television Engineers,” Vol. 52, No.11, pp. 1539–1545 and pp. 1658–1665 (1998).
As an example of the prior art, the UHF-band terrestrial digital broadcasting system in Japan will be described below. It should be noted however that this scheme involves an extremely complicated configuration, so that the following description will be made on the system which is simplified to such an extent that is required for understanding the present invention.
Beginning with description on the structure of a carrier in this broadcasting system, as illustrated in FIG. 6, this system uses a total of approximately 1,400 carriers within a frequency band W which is divided into 13 segments such that information codes of up to three channels (three layers) can be simultaneously transmitted. In a case that the information codes for three channels are transmitted, for example, about 470 carriers are used for each channel.
In this event, the number of segments and a modulation scheme used in each layer can be freely selected from several modes as shown in the above-mentioned documents. Within such selectable modes, a mode in which carriers of all segments are modulated in accordance with the same synchronous modulation scheme such as 64 QAM can be applied as it is to other transmitters such as FPU (Field Pickup Unit).
Now, referring to FIG. 7, a prior art OFDM system based on the synchronous modulation as mentioned will be described below in greater detail for an example in which carriers of all segments are modulated in accordance with the same 64 QAM scheme to transmit information codes on one layer. FIG. 7 is a diagram representing the structure of the carriers of segments which are modulated in accordance with the synchronous modulation scheme, and only shows a low end region of the frequency band W in FIG. 6.
In a mode which uses all segments for transmission of information codes on one layer, it may be thought that a similar structure is repeated over the entire band.
In FIG. 7, the horizontal direction represents the frequency; the vertical direction represents the lapse of time; and squares “□” arrayed in the horizontal and vertical directions each represent one carrier. Thus, one column of squares “□” arranged in the horizontal direction within the whole frequency band represents one symbol which forms part of an OFDM signal.
Further, a carrier “□” with “SP” denoted within the square represents the position at which a carrier is inserted for a pilot signal used for reproducing a reference signal (a reference for an amplitude and phase) during demodulation, while a carrier without any notation within the square represents a carrier for a signal of an information code modulated in accordance with the 64 QAM scheme. As can be seen in FIG. 7, since the pilot signals are scattered in the frequency direction and the time direction, they are designated as “SP” (Scattered Pilot).
It should be noted that FIG. 7 merely indicates the positioning of the pilot signals SP's in a schematic form and omits a TMCC (Transmission and Multiplexing Configuration Control) carrier which is generally included for transmitting control signals.
Also, in the actual terrestrial digital broadcasting system, every third carrier columns, in the direction of the frequency axis, have SP's arranged in the direction of the time axis, whereas the structure shown in FIG. 7 is modified such that every fifth carrier columns have SP's. This modification is made for the ease of understanding the description of the present invention, later given, and there is essentially no change in contents.
Then, a signal in accordance with the 64 QAM scheme is indicated by any of 64 signal points represented by broken line circles on an orthogonal coordinate plane in FIG. 8, where each signal point is corresponded to a sequence of codes comprised of six bits, different from one another. For example, a signal point b on the I-Q complex plane in FIG. 8 is corresponded to a code “011111”.
The modulation processing in accordance with the 64 QAM scheme involves dividing a sequence of input information codes in units of six bits, assigning each of the divided 6-bit codes to any one of the 64 signal points on the I-Q complex plane. Each of the 6-bit codes is converted to a signal corresponding to the coordinate of I-Q complex plane representing a signal point indicated by a solid line circle “◯” in FIG. 8, and outputting the converted signal. Then, a modulated signal indicative of the code is outputted.
On the other hand, the transmission signal is affected by noise and other interference during a transmission process and distorted. For this reason, for example, a solid line circle “◯” representative of a signal b, when transmitted, in FIG. 8 will move to a position indicated by a cross “X” representative of a signal point b′ when received.
The demodulation processing in accordance with the 64 QAM scheme involves selecting the signal point “b” closest to the signal point b′ for the received signal represented by the cross “X”, from 64 QAM signal points indicated by broken line circles, and outputting a 6-bit code corresponding to the selected signal point. Therefore, the demodulation processing requires the knowledge of the correct signal point position indicated by the broken line circle associated with the received signal.
The reproduction of a correct signal position only requires to find, for example, the direction and magnitude of a reference signal vector which represents the correct position of a coordinate point “a” on the signal space in FIG. 8 which serves as a reference. The direction and magnitude of the reference signal vector for a received signal, however, have been affected by multipath and so on, which may occur in the transmission system, causing the phase to rotate and the amplitude to change as shown in FIG. 9. On the reception side, the reference signal vector must be reproduced based on a pilot signal SP for each carrier. For a carrier without pilot signal SP, the reference signal vector therefor is produced based on pilot signals of neighboring carriers.
Here, while the phase and magnitude of the reference signal vector change every time or from one carrier to another, as described above, the manner of changing is generally expressed by a smooth curve and has a remarkable correlation in the time direction and in the frequency direction. For this reason, the reference signal vector for a modulated signal A of an arbitrary carrier of an arbitrary symbol in FIG. 7 can be readily found by interpolation of a plurality of sporadically transmitted SP signals. FIG. 7 shows exemplary positions of SP signals which facilitate efficient interpolation.
Though not particularly defined for the terrestrial digital broadcasting system, a method of reproducing the reference signal vector from a signal having the carrier structure shown in FIG. 7 may be implemented, for example, using a circuit illustrated in FIG. 10.
FIG. 10 illustrates an exemplary circuit for use in reproducing a reference signal vector on the reception side in an OFDM-based transmission system. In the illustrated circuit, a received signal is first outputted from an FFT (Fast Fourier Transform) circuit 30, and commonly inputted to a time direction interpolation circuit 31 and a delay circuit 32.
Then, the time direction interpolation circuit 31 first extracts SP signals from the received signal, and filters every carrier column including SP's in the time direction, as indicated by hatchings in FIG. 11, using a low pass filter (LPF) having a predetermined number of taps, to output the resulting signal as a reference signal vector signal interpolated in the time direction. Though the digital LPF is not illustrated, it is included in the interpolation circuit 31. FIG. 11 is a diagram which shows in greater detail the carrier structure of a segment modulated in accordance with the synchronous modulation scheme, in the same manner as FIG. 7. In the following, a carrier in which SP is located is designated as the “SP carrier”.
Next, FIG. 12 is a diagram schematically showing a method of determining a reference signal vector for a carrier without SP through the foregoing interpolation of SP's in the time direction. Such carriers are indicated by a one-dot chain line 22 in FIG. 7. In FIG. 12, the horizontal axis represents a time axis on which the scaling is marked for each symbol, and a vertical line with a circle added at the top represents a signal vector for a received SP.
Then, reference signal vector signals existing, for example, in a period from the receipt of certain SP, for example, SP1 to the receipt of SP2 are found by interpolation using vector signals for a plurality of SP's at positions temporally preceding to and subsequent to these symbols by means of an LPF having a fixed number of taps.
Therefore, by the interpolation of SP's in the time direction, reference signal vectors are calculated for all carriers on the columns indicated by hatchings in FIG. 11. In this event, the LPF-based operation requires signals of a number of symbols equal to the number of taps, and interpolated signals are outputted with a delay of a number of symbols corresponding to approximately one half of the number of taps. Therefore, the delay circuit 32 is provided such that the timing of the interpolated signal matches the timing of the received signal.
On the other hand, a reference signal vector for a modulated signal A on a carrier in a column in which no SP is located is found by interpolating reference signal vectors for SP carriers in a column indicated by hatchings in the frequency direction. For this reason, an output signal of the time direction interpolation circuit 31 is further inputted to the frequency direction interpolation circuit 33 which reproduces a reference signal vector signal through interpolation in the frequency direction.
FIG. 13 is a diagram schematically showing a frequency direction SP interpolation method for symbols along a one-dot chain line shown in FIG. 11. In FIG. 13, the horizontal axis represents the frequency with the scaling marked for each carrier position, and bold vertical arrows represents reference signal vectors W(1), W(5+1), W(2×5+1), . . . , for carriers which are found through the interpolation in the time axis direction in FIG. 12, i.e., the carriers indicated by hatchings in FIG. 11, where numbers in parenthesis represent carrier numbers.
In FIG. 13, a reference signal vector for a carrier position A without bold arrow is calculated in the following manner.
Specifically, in FIG. 13, assuming that the magnitude of a vector for a carrier without bold arrow is zero, and signals W(1), 0, . . . , 0, W(5+1), 0, . . . 0, W(2×5+1), . . . are processed, for example, by a general digital LPF having 23 taps to calculate smoothly interpolated signals as indicated by a broken line. Though the digital LPF is not illustrated, it is contained in the SP interpolation circuit 33.
It is therefore possible to correct a displaced signal point position in a manner shown in FIG. 9 to a correct position shown in FIG. 8 to demodulate a correct information code by inputting the reference signal vector signal reproduced by the frequency-direction interpolation circuit 33 to a 64 QAM demodulator circuit 34 together with the received signal which is delayed by the delay circuit 32 by the number of symbols equal to approximately one half of the number of taps in the LPF to correct the phase and amplitude of the received signal.